Hydrogen generator

ABSTRACT

A hydrogen gas generator generates hydrogen gas by mixing two reactants. The generator has a reaction chamber for receiving a solid reactant. The chamber has a reaction product separator impermeable to the solid reactant and a biasing means for biasing reactant products against the separator. The generator also has a liquid reactant dispenser for storing a liquid reactant and is fluidly coupled to the reaction chamber, such that dispensed liquid reactant reacts with the solid reactant in the reaction chamber to produce hydrogen gas and a waste product that are substantially permeable through the separator. The generator also has a product collector coupled to the reaction chamber for collecting hydrogen gas and waste product that have passed through the separator.

FIELD OF THE INVENTION

This invention relates generally to hydrogen generation and inparticular, to a hydrogen generator that generates gaseous hydrogen bymixing at least two reactants.

BACKGROUND OF THE INVENTION

Modern portable electronic devices are demanding increasing amounts ofelectrical power and chemical batteries are often the performancebottleneck for such devices. Wireless products, such as personal digitalassistants, mobile phones, entertainment devices, and next generationlaptops in particular have a great demand for sustained power. Forlong-term portable operations, fuel cells are an attractive solution.Fuel cells, like batteries, efficiently convert chemical energy intoelectricity, but have additional advantages, such as higher energydensity and the capability for instant refueling. Fuel cells aretypically fuelled by hydrogen gas, but there are technologicalchallenges in storing and delivering hydrogen gas to the fuel cells in acost effective and efficient manner. One particular challenge is toprovide a fuel supply that is inexpensive, safe, light and compactenough to be readily portable yet store enough hydrogen to provide auseful amount of fuel to the fuel cell. State of the art means forstoring hydrogen include metal hydride canisters to store hydrogen atrelatively low pressures, and pressure tanks to store compressedhydrogen at elevated pressures. Both approaches have drawbacks; forexample, metal hydride storage is relatively safe but has a low energydensity to weight ratio, and compressed hydrogen storage can have a highenergy density to weight ratio but requires high strength and expensivecontainment solutions.

Research has been conducted into using liquid methanol as a fuel anddesigning a “direct methanol” fuel cell that electrochemically produceselectricity directly from methanol; however, significant technologicalchallenges exist such as preventing methanol cross-over through theelectrolyte membrane, and preventing catalyst poisoning by the methanolfuel.

Other efforts have been directed at generating hydrogen gas from ahydrogen-containing fuel solution such as sodium borohydride. In suchapproaches, the fuel solution is exposed to a catalyst to facilitate theproduction of hydrogen gas. While this approach is promising,technological challenges exist in containing the caustic fuel solutionand preventing leakage, especially when the portable fuel cell systemwill be used in close proximity to humans.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved means forgenerating hydrogen gas. According to one aspect of the invention, thereis provided a hydrogen gas generator that generates hydrogen gas bymixing at least two reactants. The generator can be a fuel cartridgeespecially useful for supplying hydrogen gas to a fuel cell system. Thegenerator has a reaction chamber for receiving a solid reactant. Thechamber has a reaction product separator impermeable to the solidreactant and a biasing means for biasing reactant products against theseparator. The generator also has a liquid reactant dispenser forstoring a liquid reactant and is fluidly coupled to the reactionchamber, such that dispensed liquid reactant reacts with the solidreactant in the reaction chamber to produce hydrogen gas and a wasteproduct that are substantially permeable through the separator. Thegenerator also has a product collector coupled to the reaction chamberfor collecting hydrogen gas and waste product that have passed throughthe separator.

The generator can further comprise a hydrogen gas separator located inthe product collector and which is permeable to hydrogen gas andimpermeable to the waste product. This separator, for example, can be agas separation membrane.

The biasing means can be a spring and the solid reactant can be sodiumborohydride powder. In particular, the sodium borohydride powder can becompacted into a pill form, and the spring can apply pressure on thepill against the separator. The separator can be a screen having a meshsize that is smaller than the sodium borohydride grain size.

The liquid reactant can be an acidic solution, such as a citric acidsolution. In particular, the solution can have a pH of less than 2.

An outer shell can be provided that encloses the reaction chamber,liquid reactant dispenser and product collector; at least part of theshell is sufficiently transparent to view the amount of solid reactantremaining in the generator, thereby acting as a fuel gauge for thegenerator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a hydrogen generating fuel cartridge having afuel cell connector sub-assembly.

FIG. 2 is a partially disassembled view of a fuel cell system having aplanar fuel cell stack, a control module, and a connector and pumpsub-assembly for coupling to the fuel cartridge sub-assembly and pumpingfluid inside the fuel cartridge.

FIG. 3 is a schematic diagram of fluid flow inside the fuel cartridge.

FIG. 4 is a partially transparent side view of the fuel cartridge.

FIG. 5 is a schematic, partially transparent view of the connector andpump sub-assembly and the fuel cartridge connector sub-assembly.

FIG. 6 is a schematic exploded perspective view of the fuel cartridgeconnector sub-assembly.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to one embodiment of the invention and referring to FIG. 1, aportable fuel cartridge 10 generates hydrogen gas for use as fuel by afuel cell. Referring to FIG. 2, a portable fuel cell system 12 comprisesfuel cells 14 which generate electricity by electrochemically reactinghydrogen gas and oxygen in ambient air. The by-products of theelectrochemical reaction also include water and heat. The generatedelectricity can be used to power portable electrical devices, and toprovide heat. The portable fuel cartridge 10 has a connectorsub-assembly 15 for physically and fluidly coupling the fuel cellcartridge 10 to the fuel cell system 12. Once coupled, hydrogen gasgenerated by the fuel cartridge 10 can be delivered into the fuel cellsystem 12 for use by the fuel cells 14.

One particular use contemplated for the fuel cartridge 10 and fuel cellsystem 12 is to provide heat and electrical power to personal apparel,such as a jacket. The fuel cell system 12 shown in FIG. 2 isparticularly suited for such use. Each fuel cell 14 is arranged in aplanar array and electrically connected in series to form a stack. Thefuel cells 14 are embedded in a spaced manner within a flexible foam andfabric laminate frame 16. Flexible fuel conduits 18 and electricalconductors 20 interconnect each fuel cell 14. The fuel outlet of onefuel cell 14 is fluidly coupled to the fuel inlet of the adjacentdownstream fuel cell 14 by the fuel conduits 18. The fuel cell stack hasa dead ended fuel flow design, in which the last fuel cell 14 is coupledto a purge valve, which can be periodically opened to dischargecontaminants and water in the fuel cell stack. The first fuel cell 14 isfluidly coupled to a pumping and connector sub-assembly 22, which isprovided with means for fluidly and physically coupling to the connectorsub-assembly 15 of the fuel cartridge 10. A control system 23 forcontrolling the operation of the fuel cell system is electricallycommunicative with the pumping and connector sub-assembly 22 to controloperation of the pump, the purge valve, a voltage sensor and pressuresensor (not shown) coupled to electrical conductors 20, and, a userinterface controls and display (not shown).

The fuel cells 14 are planar, passive air breathing proton exchangemembrane (PEM) type fuel cells. Each fuel cell comprises a conventionalplatinum catalyst-coated electrode and Nafion™ membrane electrodeassembly (MEA), sandwiched by cathode and anode assemblies (not shown).The cathode assembly comprises a conductive mesh in adjacent contact tothe cathode side of the MEA, and a conductive plate with multipleopenings there through in adjacent contact with the conductive mesh; themultiple openings are exposed to air and provide access to oxygen usedin the electrochemical reaction. The anode assembly comprises aconductive anode plate with serpentine flow channels in adjacent contactwith the anode side of the MEA, and a hydrogen gas manifold plate andhaving hydrogen inlet and outlet and manifolds that fluidly couple toinlet and outlet ends of the anode plate fuel flow channels. The inletand outlet manifolds fluidly couple to respective inlet and outlet fuelconduits 18. The MEA also features adhesive around its periphery, andwith the adhesive layer, bonds the fuel cell components together.

The fuel cell stack 12 in this embodiment is configured to provide about10 watts of power; however it is within the scope of the invention toscale up or down the power output by changing the number of fuel cells,or substituting fuel cells of different performance ratings.

Such planar PEM fuel cells 14 are well known in the art and are notdescribed in any further detail here. While the fuel cell system 12 isparticularly suited for PEM fuel cells, other fuel cell types that arefuelled by hydrogen gas can be substituted, such as solid oxide fuelcells, phosphoric acid fuel cells and alkaline fuel cells. Also, otherknown PEM fuel cell designs can be readily substituted.

Because the fuel cells 14 are embedded in the flexible frame 16 and areinterconnected by flexible conduits 18 and electrical connectors 20, theshape of fuel cell system 12 can be changed; this feature isparticularly advantageous for use in apparel, as the fuel cell system 10can conform to the shape of the wearer. Preferably, the fuel system 12is installed along the upper spine region of the jacket, so that thefuel cells 14 in the stack can conform to the shape of the wearer'sback. However, it is within the scope of the invention for the fuel cellsystem 12 to assume different configurations, e.g. a conventionalvertically arranged stack. In such alternative configurations, the fuelcells in the stack would not necessarily be flexibly interconnected, andinstallation of such fuel cell stacks in apparel would be modified toprevent discomfort to the user. Also, while five fuel cells 14 are shownin this embodiment, it is within the scope of the invention to scale upor down the number of fuel cells and the corresponding power outputdepending on the particular application and power need.

Two layers of soft flexible foam are used to sandwich the gas, current,and voltage sensing interconnects within the fuel cell system. Theseparts hold the system interconnects in place and provide strain reliefagainst mechanical forces (bending, stretching, etc.) put on the system.Additionally, they provide a lightweight covering for theinterconnecting elements (wire, tubes, voltage sensing wires) that hidesthese parts from the user and creates a soft, body friendly packaging.

In the current embodiment, a wicking type fabric is laminated over theoutside of the system covering a plane including the cathodes of thefuel cells 14. This material is designed to rapidly evaporate anymoisture in contact with it. Placing this material in contact with thecathode enables a rapid evaporation of any moisture that collects on thecathode of the fuel cell 14, reducing the risk of flooding in the cells14. Covering the entire system 12 with this fabric maximizes the surfacearea for evaporation. Additionally, this fabric serves as a flexiblestrain-relieving interconnect between the multiple fuel cells 14 in thesystem 12. Lastly, this fabric creates a surface texture for the fuelcell system 12 that feels soft and pleasant when used close to the skin,making the product more comfortable for near-body applications.

Preferably, the fuel cartridge 10 is constructed from lightweight andinexpensive materials to enable the fuel cartridge 10 to be easilyportable and disposable after a single use. In this embodiment and asshown schematically in FIG. 3, the fuel cartridge 10 stores a liquidreactant, namely, a 28 wt. % citric acid solution, in an outer bag 25and a solid reactant, namely, a compacted and fused sodium borohydride(NaBH₄) powder, in a tubular reaction chamber 26. The reaction chamber26 (Teflon tubing, 0.60″ ID, 0.030″ wall, McMaster) is stored within aninner bag 27 that is fluidly sealed from the solution bag 25. The NaBH₄powder is compacted into a cylindrical pill 28, and a spring 29 insidethe reaction chamber 26 biases the pill 28 against an outlet having aseparator screen 30 at one end of the reaction chamber 26. In thecurrent embodiment, a 0.5″ diameter pill was formed by pressingapproximately 12 grams of Sodium Borohydride powder under 7 tons offorce to form a pill 3.2 inches in length.

The reaction chamber 26 should be constructed of a material that canboth withstand the heat of the reaction, which can lead to temperaturesin excess of 170° F. and will allow the pill 28 to slide under the forceof the spring without binding. Teflon or polyethelyne both meet theserequirements suitably.

The separator screen 30 has a mesh size that is smaller than thepowder's grain size, thus preventing the pill 28 from exiting throughthe outlet, but allowing liquid, gas and particulates smaller than thescreen openings to flow there-through. Plastic mesh with a screen pitchof 0.080″ and strand size of 0.005″ is used in the current embodiment.

When the fuel cartridge 10 is coupled to the pump and connectorsub-assembly 22 of the fuel cell system 12, acid solution can be pumpedfrom the outer bag 25 through a pumping chamber 52 inside the connectorsub assembly 15 and into the reaction chamber 26 near the separatorscreen 30. When the acid solution and NaBH₄ mix, hydrogen gas and awaste slurry is formed; the pressure of the spring 29 forces the gas andslurry through the separator screen 30 and into a product collectionportion of the inner bag 27 (“product collector” 31). The slurrycomprises solids suspended in liquid, and in particular, is a mixture ofsodium metaborate, water, and a salt of an acid, and has a benignacidity of around pH 7. The particular size of the slurry solids shouldbe smaller than the mesh size of the separator screen 30 so that theslurry solids can pass there-through. At the downstream end of theproduct collector 31 is a hydrogen gas separation membrane 32, which ispermeable to hydrogen gas but impermeable to liquid and solid. Hydrogengas is separated from the slurry and delivered to fuel cell system 10via an outlet port 40 in the connector sub-assembly 15.

FIG. 4 illustrates the construction of the fuel cartridge 10 in greaterdetail. The inner bag 27 is located inside the outer solution bag 25,such that the acid solution resides in the volume in between thesolution bag 25 and inner bag 27. In the current embodiment, 6 milurethane (Stevens Urethane, East Hampton, Mass., US) was heat welded toform the inner and outer bags 25, 27. A solution feed conduit 33 fluidlycouples this volume to the pumping chamber 52, and also extends from thepumping chamber 52 through the inner bag at opening 34 and to a nozzle35 in the reaction chamber 26 near the separator screen 30. The nozzle35 is an elongated tube (stainless steel hypodermic tubing with thedistal end crimped and sealed and an orifice perpendicular to thecentral axis of the tube and facing the central axis of the pill 28,McMaster) that spans the diameter of the reaction chamber 26. Solutionis discharged through the hole and contacts the pill 28. The distancebetween the nozzle and the separator screen 30 allows for mixing of thereactants prior to leaving the reaction chamber 26. Several alternativesknown in the art exist for ensuring proper mixing of the reactants andproducts in order to maximize the yield of the reaction and the energydensity of the cartridge 10. For example, the cartridge 10 can use asingle nozzle or a plurality of nozzles (not shown) to help ensure moreeven mixing of the reactants. These nozzles could be a circular orlinear array with orifices designed to mist, spray, or provide dropletsto the pill 28. A distance and preferably tortuous path between thenozzle and the screen 30 allows for thorough mixing of the reactantsprior to leaving the reaction chamber 26. In the current embodiment, thenozzle 35 is 0.4″ from the separator mesh, although the optimum distancewill vary with the nozzle design, flow rates, etc. An additional featurethat has been found to be advantageous in the reaction area design is toconstruct the nozzle 35 such that the reacting pill can form around thenozzle 35 as it is reacted by the solution from the nozzle 35. Thisensures intimate contact between the nozzle 35 and the pill 28 and areasonably tortuous path for the fluids before they exit the reactionchamber 26. An additional advantage of the current embodiment in whichthe reaction area is limited to the end of the pill is that the heat ofreaction can be contained in a relatively small space, maximizing thetemperature of the reaction area. This higher temperature has favourableeffects on the reaction efficiency, kinetics, and the ability to restartthe system 12 with reaction products collected and hardened about theseparator screen 30.

As the spring 29 is applying continuous pressure on the pill 28 againstthe separator screen 30, product hydrogen gas and waste slurry aredischarged through the separator screen 30 and into the productcollector 31, which has a serpentine flow path formed by two generallystraight welds 36 which join the inner bag surfaces together. The weldsform a central pocket in which the cylindrical reaction chamber 26 islocated. The product collector 31 is partially filed with a liquidabsorbing material 37 for absorbing water and other liquid in theslurry. This material minimizes the contact of the slurry with the gascollection membrane, as the slurry tends to form an impermeable coatingon the membrane after prolonged exposure. Although higher performancematerials exist, the highly absorbent material found in tampons wasfound to perform suitably for this application. Unabsorbed slurry andhydrogen gas continue along the product collector 31 to the gasseparation membrane 32. Hydrogen gas flows through the separationmembrane 32 (Versapore 3000 (Pall, Ann Arbor, Mich.) and into a hydrogendelivery tube 38, which is coupled to the separation membrane 32 andextends through the inner bag at opening 39 and couples to the dischargeport 40 in the connector sub-assembly 15. The flow path of the solution,waste slurry and hydrogen gas are illustrated by arrows in this figure.

While mixing water alone with the NaBH₄ is sufficient to chemicallyproduce hydrogen gas, the reaction rate is slow. Preferably, the acid isprovided to speed up the reaction rate; in this sense, the acid actslike a catalyst, although the acid is consumed in the reaction. While inthis embodiment, 28 wt. % citric acid solution is reacted with NaBH₄powder to generate hydrogen, any acid solution with a suitable pH can besubstituted. Preferably, the acid solution has a pH of 6 or less; morepreferably, the acid solution has a range of 2 or less. The 28 wt. %citric acid solution has a pH of about 2. This concentration was foundto provide a desirable balance of low pH, fast rate of reaction, andminimal wastage of acid. That is, substantially all of the acid in thesolution was consumed in the reaction. When selecting alternative acids,such a balance is also desirable.

Alternative liquid and solid reactants that produce hydrogen gas whenmixed can be substituted. For example, tests have shown that the fuelcartridge 14 can mix aluminium with sodium hydroxide solution to producehydrogen gas, in the following reaction:

2Al+2NaOH+6H2O---→2NaAl(OH)4+3H2  (1)

It is expected that other known reactions between reactants that producehydrogen gas can be used in the fuel cartridge 10, provided that one ofthe reactants can be stored in compacted solid form, an another of thereactants can be separately stored in liquid form. The fuel cartridge 14design is particularly effective for facilitating such reactions, as aportion of the solid is continuously exposed to the liquid reactant,since the biasing force provided by the spring 29 forces the gaseous,liquid and small particulate products of the reaction through theseparator screen 30. This prevents the products of the reaction fromcoating the solid reactant, and from mixing with the liquid reactant.Care should be taken in selecting the solid powder grain size andseparator screen size so that the solid reactant is not pushed throughthe separator screen 30.

While a spring 29 is used to provide a biasing force against the solidreactant, other biasing means can be provided. For example, the reactionchamber 26 can be a single-ended flexible sheath that is stretches whenfilled with the solid reactant, and applies pressure on the solidreactant towards the sheath's opening. Other equivalent biasing meanscan be readily substituted. In the embodiment shown in these Figures,and referring particularly to FIG. 1, the biasing spring 29 appliespressure against the pill 28 such that a portion of the pill 28 isalways pressed against the separator screen 30. As the pill 28 isconsumed, the spring 29 will expand; a clear window 41 is provided inthe surface of the fuel cartridge 10 such that the amount of pillmaterial is visible. This window 41 serves as a fuel gauge to displaythe amount of reactant left in the fuel cartridge 10.

Referring now to FIGS. 5 and 6, the cartridge connector sub assembly 15protrudes from the fuel cartridge outer shell and can be connected tothe pump and connector sub-assembly 22 of the fuel cell system 14. Thepump and connector sub-assembly 22 has a recess 43 adapted to receivethe protruding connector sub-assembly 15. A pair of magnets 44 areprovided in the base of the connector sub-assembly 15, and are attractedto metal plates in the recess 43. The magnets 44 provide a means forsecuring the fuel cartridge 10 to the fuel cell system 12; however,other securing means as known in the art can be substituted within thescope of the invention.

The pump and connector sub-assembly 22 is provided with a hydrogenintake port 46 that mates with the hydrogen discharge port 40 when thefuel cartridge 10 is connected to the fuel cell system 12. A pumpplunger 48 extends from the recess 43 of the pump and connectorsub-assembly 22; a boss 49 protrudes from sub-assembly 22 around pumpplunger 48, protecting the plunger 48 from damage when the connectorsub-assembly 15 is not in place. A diaphragm port 50 is provided in theconnector sub-assembly 15 that receives the pump plunger 48 when thefuel cartridge 10 and fuel cell system 12 are connected. The diaphragmport 50 extends into a bottom end of a pumping chamber 52. At a top endof the pumping chamber 52 are an acid solution inlet 56 and an acidsolution 58. A flexible diaphragm 54 is mounted inside the pumpingchamber 52 and fluidly seals the pumping chamber volume from thediaphragm port 50; this confines the flow of acid solution from theinlet 56 into the pumping chamber 52 and out of the outlet 58. A biasingspring 60 is located in the second chamber portion and applies a biasingforce against the diaphragm 54 to bias the diaphragm 54 in an unflexedposition.

The acid solution inlet and outlet 56, 58 are fluidly coupled to thesolution feed conduit 33.

The connector sub-assembly 22 has an outer shell comprising two moldedplastic portions: an outer shell portion 62 contains a portion of thepumping chamber 52, the diaphragm port 50, hydrogen discharge port 40,and magnets 44. An inner shell portion 66 contains the rest of thepumping chamber 52, solution inlet 56 and outlet 58; the diaphragm 54 isfixed in place between the outer and central portions 62, 64. The shellportions 62, 66 are joined by LokTite 3105 Light Cure Adhesive. Whilethe sub-assembly 22 is formed by joining together these shell portionsby an adhesive, it is to be understood that many other methods known inthe art are available for creating this sub-assembly 22.

When the fuel cartridge 10 and fuel cell system 12 are connected, adistal end of the pump plunger 48 extends through the diaphragm port 50,into the pumping chamber 52 and contacts the diaphragm 54. The pumpplunger 48 is slidably constrained within the pumping and connectorsub-assembly 22 in an axial direction and between a fully extendedposition and a fully retracted position. When the pump plunger 48 is inits fully retracted position, it makes contact with but does not flexthe diaphragm 54, i.e. the diaphragm 54 is in its unflexed position.When the pump plunger 48 is in its fully extended position, thediaphragm 54 is moved by the plunger 48 into its flexed position. Thereciprocating movement of the pump plunger 48 causes the diaphragm 54 tooscillate, thereby creating a pumping pressure within the pumpingchamber 52. This pumping pressure is effective to pump the citric acidsolution from the solution bag 25 to the reaction chamber 26.

Recpriocating movement of the pump plunger 48 is achieved by contractionand extension of a shape memory alloy wire 70 connected to a plungerhead 72 located at the proximal end of the pump plunger 48. The shapememory alloy wire 70 is comprised of a shape memory alloy material, suchas a nickel-titanium alloy popularly known as “nitinol”. The shapememory alloy material is sensitive to temperature or heat. For example,nitinol temporarily shrinks at a range of temperatures dictated by thecomposition of the nitinol; in this embodiment, the nintol wire 70shrinks at about 100° C. The nitinol alloy will expand at a relativelower temperature and return to its original condition. In response tobeing heated above this shrinkage temperature, the nitinol alloyundergoes a dimensional change, such as a change in its length. In thisway, the nitinol wire 70 can undergo a reduction in length and return toits original length repeatedly via repeated temperature cycling aboveits shrinkage temperature and cooling to below its expansiontemperature.

It in the process of undergoing a dimensional change, as describedabove, the shape alloy material goes through a reversible phasetransition or transformation, or a reversible structural phasetransition, upon a change in temperature. Generally, such a transitionrepresents a change in the material from one solid phase of the materialto another, for example, by a change in the crystal structure of thematerial or by a re-ordering of the material at a molecular level. Inthe case of the nitinol wire 70, the superelastic alloy has a lowtemperature phase, or martensitic phase, and a high temperature phase,or austenitic phase. These phases can also be referred to in terms of arelaxed phase and a soft and malleable phase, or contracted phase.

The nitinol wires 70 is threaded through the plunger head 72 andattached at either end to the pumping and connector sub-assembly 22 bycrimp connections 74. The nitinol wire 70 is located such that when inits relaxed phase, the plunger head 48 is in its retracted position;when the nitinol wire 70 is in its contracted phase, the plunger head 48is in its fully extended position. The crimp connections 74 areconnected to electrical wire (not shown) that is electrically coupled toa rechargeable battery (not shown) located in the control unit 23. Thebattery in turn is electrically connected to the electrical connectors20 such that the battery can be recharged by electricity produced by thefuel cells 14. Electrical current through the nitinol wire 70 from theelectrical wires will result in heating of the nitinol wire 70 above itsshrinkage temperature, thereby causing the plunger 48 to move from itsretracted position to its extended position, i.e. execute a compressionstroke. When the plunger 48 reaches its fully extended position, theplunger head makes contact with a detector switch 76, which iselectrically communicative with and sends a signal to the control unit23. Upon receipt of this signal, the control unit 23 stops current flowfrom the battery or fuel cell system, and the wire 70 is allowed to cooland fall below its expansion temperature. Alternatively or additionally,the pulse length of the current provided to the wire 70 can becontrolled by methods known in the art such that the wire is heatedenough to cause it to contract. The wire 70 expands to its originallength, and the plunger 48 is moved back into its fully retractedposition, i.e. execute an expansion stroke. The frequency of the plungerstrokes is dictated by the amount of hydrogen gas required; when moregas is required, more solution needs to be pumped to the reactionchamber 26, and the frequency of the plunger strokes is increased.

By locating certain pumping components, i.e. nitinol wire 70, plungerhead 72, plunger 48 outside of the fuel cartridge 10, the manufacturingcost of the cartridge 10 is reduced. Also, by sealing the pumpingchamber 52 with the diaphragm 52, the citric acid solution is notpermitted to leave the fuel cartridge 10; this design minimizes thelikelihood of damage or harm caused by acid leakage. The only fluid thatis permitted to leave the fuel cartridge 10 is hydrogen gas, via port40. A further advantage offered by this design is the simplified controlof gas generation. Since hydrogen gas is generated only when the citricacid solution is mixed with the solid NaBH₄, the rate of pumpingentirely controls the rate of hydrogen gas production.

Although the present invention and its advantages have been described indetail, it should be understood that the present invention is notlimited to or defined by what is shown or discussed herein. Thedrawings, descriptions and discussions herein show examples of theinvention and provide examples of using the invention. One skilled inthe art will realize the implementations of the present invention couldbe made without departing form the principles, spirit or legal scope ofthe present invention. Accordingly, the scope of the present inventionshould be determined by the following claims and their legalequivalents.

1. A hydrogen gas generator comprising: a reaction chamber for receivinga solid reactant, the chamber having a reaction product separatorimpermeable to the solid reactant and a biasing means for biasingreactant products against the separator; a liquid reactant dispenser forstoring a liquid reactant and fluidly coupled to the reaction chamber,such that dispensed liquid reactant reacts with the solid reactant inthe reaction chamber to produce hydrogen gas and a waste product thatare substantially permeable through the separator; and, a productcollector coupled to the reaction chamber for collecting hydrogen gasand waste product that have passed through the separator.
 2. A hydrogengas generator as claimed in claim 1 further comprising a hydrogen gasseparator located in the product collector and which is permeable tohydrogen gas and impermeable to the waste product.
 3. A hydrogen gasgenerator as claimed in claim 2 wherein the hydrogen gas separator is agas separation membrane.
 4. A hydrogen gas generator as claimed in claim1 wherein the biasing means is a spring.
 5. A hydrogen gas generator asclaimed in claim 1 wherein the solid reactant is sodium borohydridepowder.
 6. A hydrogen gas generator as claimed in claim 5 wherein thesodium borohydride powder is compacted into a pill form.
 7. A hydrogengas generator as claimed in claim 6 wherein the separator is a screenhaving a mesh size that is smaller than the sodium borohydride grainsize.
 8. A hydrogen gas generator as claimed in claim 1 wherein theliquid reactant is an acidic solution.
 9. A hydrogen gas generator asclaimed in claim 8 wherein the acid solution has a pH of 2 or less. 10.A hydrogen gas generator as claimed in claim 1 wherein the liquidreactant is citric acid solution.
 11. A hydrogen gas generator asclaimed in claim 1 further comprising an outer shell enclosing thereaction chamber, liquid reactant dispenser and product collector,wherein at least part of the shell is sufficiently transparent to viewthe amount of solid reactant remaining in the generator.
 12. A hydrogengas generator as claimed in claim 1 wherein the liquid reactantdispenser is a bag, and the generator further comprises an inner bagenclosing the reaction chamber and product collector, the inner bagbeing located within the dispenser bag.